the death of the test-organism is recognized as the most authentic indicator 

 of the toxic action of a substance. 



It is a criterion which is beyond the concept of normalcy and pathology, 

 since death represents a leap to a new quality to which no characterizable 

 biological concepts can be applied. In this case the biological essence of 

 death is disregarded, and the result of an experiment is considered as 

 simply the answer to the question: is the substance toxic or not? But at 

 the population level, the essence of this question is different. The LC50 

 criterion itself means that any population is heterogeneous in relation to 

 its sensitivity to the toxicant. It suggests that there are resistant and 

 tolerant individuals within it, and, therefore, the toxicant functions as a 

 factor of natural selection with regard to the fate of the population. 



Mortality as an ecological and evolutional factor controlling population 

 numbers has appeared together with life, and it would disappear only 

 together with it. If death means an awful and final defeat in the struggle 

 for existence for an individual, then for a population mass death is only 

 the elimination of the less adaptative, the survival of the more adaptative 

 incorporates some form of "reorganization", the essence of which is that 

 the population number declines abruptly first, then as resistant forms ap- 

 pear, a population numbers outbreak is observed. A health experience with 

 insecticide application is evidence of this phenomena. As a result of wide 

 utilization of strong insecticides, the insects not only survived but on the 

 contrary reproduced intensively. Aquatic animals are no exception to this 

 phenomena. It is known for instance, that mosquito fish resistant to DDT 

 have recently appeared (Holden 1973). In another case, a Cladocera test- 

 culture appeared to be killed in the V-VI-th generation under the influence 

 of toxicants, however, the XY-XYI-th generation "suddenly" revived and began 

 to breed rapidly. Finally, an algae culture almost killed under the in- 

 fluence of algaecide preparations was able to recover, and new cell genera- 

 tions grew. In principle, all these phenomena mean that the population has 

 latent resources to aid in elimination, and with the decrease of environ- 

 mental toxicant concentration, it can function as stimulative factor for re- 

 production of the organisms inhibited by it, in accordance with the law of 

 phase reactions. 



Aquatic organisms, in contrast to warm-blooded animals, have other 

 latent resources, namely the ability to survive unfavorable conditions in a 

 resting stage, i.e., the statoblasts of moss animals, turions of aquatic 

 animals, ephippia of Cladocera, spores and cysts of Protozoa, the closing of 

 mollusk, shells, and the resting stage of algae. All these forms of life 

 exist in sediments, and are not susceptible to toxic effects. The adapta- 

 tion to very severe conditions in water is a rather good protection against 

 toxic agents, and it serves to guard populations from destruction by toxic 

 substances. In contrast to poikilothermal aquatic species, homothermal or- 

 ganisms are physiologically only accessible to poisons under conditions of 

 optimal temperature. At temperatures below 15°C, their biologic processes 

 are so inhibited, and exchange with environment is so reduced the the pre- 

 sence of a toxicant in their environment is of no serious danger to them. 

 Thus, the toxicity of a substance, and the even higher values of the LC50 

 obtained in the experiments with actively functioning individuals is not 



37 



